Abstract:

Rhododendron species are host to many Phytophthora species and are an important crop in the nursery industry. The foliar epidemiology of twelve Phytophthora diseases on Rhododendron was studied in field and growth chamber experiments on two Rhododendron cultivars. The relative aggressiveness of Phytophthora spp. was assessed using disease incidence, severity, and sporulation. Plant experiments were conducted on whole plants and on detached leaves. Growth rates of lesions and colonies for each species at different temperatures were determined in growth chambers on detached leaves and in agar culture. Whole plants were inoculated in the field in four experiments conducted at three to four month intervals. The twelve species studied appeared to fall into two groups with P. citrophthora, P. foliorum, P. kernoviae, P. parasitica, P. plurivora and P. ramorum being the most aggressive species; sporulating the most, developing the largest lesions or exhibiting the highest infection rate. P. cactorum, P. cambivora, P. cinnamomi, P. lateralis, P. nemorosa and P. syringae were less aggressive on foliage under most conditions, with the exception of P. syringae which developed the largest lesions at lower temperatures typical of spring time in temperate climates. It is apparent that different Phytophthora species are more active at different temperatures and thus surveys for Phytophthora infections should be concerned about different species at different times of the year. P. kernoviae is not known to be present in the US. P. kernoviae is of particular concern due to the high sporulation levels observed in detached leaf and whole plant inoculations. Rhododendron spp. appear to be likely candidate vectors for the introduction of novel Phytophthora pathogens. This work has implications for the management of Phytophthora pathogens and importation or movement of Rhododendron plants across nurseries.